Since the release of the first hard disk drives incorporating "Winchester" technology, there has been an awareness in the industry of potential problems associated with head/media contact during power on/power off conditions. These problems generally relate to the possibility of friction-induced wear of the read/write heads or the recording medium during those intervals of time before the "air bearing" on which the read/write heads "fly" has become established during spin-up of the disks, and after the "air bearing" has deteriorated during spin-down.
Through the intervening years, many schemes have evolved to minimize the risks associated with "contact start/stop":
1. In disk drives incorporating stepper motors to move the read/write heads across the disk surface, the heads have frequently been allowed to land on the disk surface wherever they happened to be when power was removed. The detent of the stepper motor was then sufficient to "latch" the heads in this position until power was once again restored to the drive.
Philosophically, this approach relied on the random nature of the landing location to prevent head/media wear from building up to a catastrophic level. This is by far the simplest scheme to implement.
Several problems, however, are implicit in this approach:
a) Since the heads land over data on the disk surface, any fatal damage caused by head/disk contact will result in loss of stored data; PA1 b) Any damage to the read/write head caused by head/disk contact will render the head incapable of reading or writing to any area on the associated disk surface; PA1 c) When "Voice Coil Motors" (VCMs) are used to move the heads, there is no detent to prevent movement of the heads while they are in contact with the disk surface when power is removed. Clearly, any relative motion of the heads and disks while they are in direct contact should be avoided.
2. The next simplest approach is to create a "dedicated parking zone" to which the heads are moved at all power losses. This scheme is typically achieved using the back-EMF of the motor which spins the disk, or spindle motor, and is very simple to implement in VCM drives, since a simple DC current applied to the voice coil will move the heads to the inner or outer extreme of their range of motion. A method of using the back-EMF of the spindle motor to drive a stepper motor to the dedicated landing zone has been disclosed in U.S. Pat. No. 4,679,102, issued Jul. 7, 1987, assigned to the assignee of the present invention and incorporated herein by reference.
The parking zone is usually located adjacent the inner diameter of the disk surface to minimize the starting torque requirement of the spindle motor.
The dedicated parking zone typically does not contain user data, and so this approach overcomes the problem stated in 1.a) above.
However, since all starts and stops will occur in the same location using this method, the likelihood of catastrophic failure during the life of the drive is greatly increased.
Further, VCM drives using this method require the addition of a mechanical latch to hold the heads in the dedicated parking zone. Such a mechanical latch adds to the cost and complexity of the drive design.
As stated above, this approach relies on the back-EMF of the spindle motor being of sufficient size and duration to accomplish the parking maneuver.
3. Ramp Loading/Unloading of the read/write heads represents the third solution to the problem of contact start/stop, and is actually a reversion to previous technology, wherein the heads are physically removed from association with the disks at power down, and brought back into association with the disks at power up only after the disks have been spun up to full operational speed.
This approach includes the provision of some form of "ramping" mechanism adjacent the outer diameter of the disk which acts cooperatively with the load beam/gimbal assembly carrying the read/write head to mechanically lift the read/write head off the air bearing on which it flies when the head-moving actuator moves the read/write heads outward past the outermost data area Of the disk. An example of this type of mechanism is shown in U.S. Pat. No. 4,535,374, issued Aug. 13, 1985 (Amcodyne).
Again, in most cases the power to move the read/write heads to and onto the ramps is derived from the back-EMF of the spindle motor during a power-off sequence.
With the advent of hard disk drives of ever-decreasing physical size, the spindle motor has, itself, become greatly reduced in size. This reduction is size results in a greatly reduced amount of back-EMF being available to power head parking or unloading operations.
Typical 2.5 inch drives produce only about 300-500 mA of current for a time interval of approximately 2-5 seconds while the drive is spinning down during a power-down. This small amount of power, available for such a short period of time, makes the design of ramp loading and unloading mechanisms a very critical area of the overall disk drive.